The invention relates generally to an active element that is advantageously included in an electrooptical device and more particularly to an improved nonlinearly conducting metal-insulator-metal (MIM) type of active element in which the insulating layer is a nonlinearly conducting thin organic film and the active elements are arranged to form an active matrix.
Various types of conventional two-terminal and three-terminal active elements have been included in conventional electrooptical apparatuses that include an active matrix. For example, thin film transistors have been used as active elements but they are structurally complex and it is difficult to form a large a high density matrix that includes thin film transistors as the active element.
Because two-terminal active elements have a simpler structure and are easier to produce then three-terminal active elements, they can yield a less costly and simplified active matrix structure for an electrooptical apparatus. Two-terminal active elements having the simplest structure are formed by disposing a nonlinearly electrically conducting layer on a first electrically conducting layer and disposing a second electrically conducting layer on the nonlinearly electrically conducting layer. Conventionally, a metal is used as the first and second conducting layers and an insulating film is used to form the nonlinearly conducting layer. Consequently, this structure is generally referred to as a MIM (Metal-Insulator-Metal) structure and the element having the MIM structure is called a MIM element.
Throughout the application, the term "MIM" refers to a structure having a first conducting layer disposed on a substrate, a nonlinearly electrically conducting layer and a second conducting layer thereon. The types of materials and their characteristics need not be defined. The term "MIM element" represents an active element including the MIM structure.
It is known that it is advantageous to include MIM elements in electrooptical liquid crystal apparatuses. For example, the principals and advantages of such an apparatus and a method of producing the apparatus are discussed in Japanese Patent Publication No. 161273/70. The MIM element is an active element due to its nonlinear electroconductive characteristics.
FIG. 3 shows an equivalent circuit for a single picture element of an electrooptic apparatus that includes a MIM element. C.sub.MIM and R.sub.MIM represent the electrostatic capacity and the linear resistance, respectively, of the MIM element. C.sub.D and R.sub.D represent the resistance and capacity of the picture element of the liquid crystal. For proper performance of the MIM element, it is necessary that the capacity ratio "R" (C.sub.D /C.sub.MIM) not be less than 5 and preferably not less than 10. Accordingly, the electrostatic capacity of the MIM element must be small compared to the capacity of the electrooptical picture element.
The nonlinearly conducting layer of a conventional MIM element is a film of anodized metal, typically tantalum pentoxide formed by anodizing the surface of the first metal conducting layer. Since the Ta.sub.2 O.sub.5 film has a relatively high dielectric constant, there are several disadvantages for its use, as follows:
1. A large voltage must be applied to the element and it is necessary to make the electrostatic capacity less then 1/5 of that of the liquid crystal, fine processes approaching physical limits are required. PA1 2. Because the properties of the element (.beta. value of Poole-Frankel) is inversely proportioned to the square root of the dielectric constant ".epsilon." and the thickness of the film and, the liquid crystal display cannot be driven at less than 1/400 to 1/500 duty.
To minimize the capacity of the MIM element, it is necessary to minimize the effective area of the MIM element. To form such a small element, it is necessary to utilize a relatively complex and costly technique such as photolithography of a minute pattern on a large substrate and/or only use the side surface of the metal electrode that forms the first conducting layer. These procedures reduce the margins for error and the manufacturing yield and increase the price of both liquid crystal devices that include MIM elements and the machinery needed to produce them.
The nonlinearity of the electrical conduction characteristics of the MIM can be represented as .beta. from the Poole-Frankel equation. .beta. is generally inversely proportional to the square root of the dielectric constant ".epsilon." and the film thickness. Accordingly, materials having large dielectric constants also have inferior nonlinearity of electrical conduction. Consequently, the practical duty ratio at which a conventional MIM element can be driven in a liquid crystal electrooptical element is at most about 1/500.
Accordingly, it is desirable to develop an improved MIM element for an electrooptical apparatus including a nonlinear conduction layer which avoids these shortcomings of the prior art.